Fischer-Tropsch synthesis (FTS) is a set of reactions by which CO and H2 (syngas) are converted into a wide variety of hydrocarbons [Dry, M. E., Catalysis-Science and Technology, p. 160, (1980); Anderson, R. B., et al., The Fischer-Tropsch synthesis, Academic Press, Inc., NY, (1984)]. This synthesis provides the best means currently available for the conversion of natural gas and carbonaceous fuels such as coal, coke, and petroleum residue to liquids and chemicals, particularly fuel and premium waxes. When FTS is used to convert low hydrogen-to-carbon ratio solid fuels, for reforming of natural gas with CO2, or for other feedstocks producing a syngas relatively lean in hydrogen (H2/CO≅0.4 to 1.0), the use of a catalyst with water gas shift (WGS) activity is highly preferred in order to generate additional H2 during the reaction as seen below:CO+2H2→(—CH2—)n+H2O  (FTS)CO+H2O→CO2+H2  (WGS)
Iron (Fe) is the preferred catalyst for low H2/CO ratio syngas over its competitor cobalt (Co) because iron is one of the most active FTS catalysts that is also active for WGS. Iron is also much less expensive than Co and has lower methane selectivity in FTS. For these reasons, iron FTS catalysts have been the subject of extensive research focus; see, for example, Bukur, D. B., et al., Natural Gas Conversion IV, Vol. 107, p. 163, (1997); Jothimurugesan, K., et al., Natural Gas Conversion V, Studies in Surface Science and Catalysis, Vol. 119, p. 215 (1998); Jothimurugesan, K., et al., Catalysis Today, Vol. 58, p. 335, (2000); O'Brien, R. J., et al., Applied Catalysis A: General, Vol. 196, p.173, (2000); and Liaw, S. and Davis, B. H., Topics in Catalysis, Vol. 10, p. 133, (2000).
Because FTS is highly exothermic, efficient heat removal from the FTS reactor is necessary to prevent catalyst deactivation via sintering and to maintain high catalyst activity and selectivity. A slurry bubble column reactor (SBCR) is the preferred reactor type for FTS. The reactor operates with fine catalyst particles dispersed in a liquid medium and gas is sparged as fine bubbles from the reactor bottom into the liquid. The preferred liquid medium for FTS is the wax product produced in the FTS reaction itself. The wax provides an efficient heat sink and the gas bubbles provide agitation and allow the heat to be rapidly absorbed and dissipated. SBCRs have relatively simple designs and low initial costs while still permitting high catalyst and reactor productivity. Other advantages of SBCRs for FTS include the ability to use low H2/CO ratio syngas and favorable conditions for catalyst regeneration and/or makeup.
Much recent work related to slurry-phase FTS has focused on using iron catalysts. These catalysts have been prepared by precipitation to achieve high activity for FTS and high selectivity for liquid hydrocarbon and wax. Alpha (α) is the well-known Anderson-Schulz-Flory chain growth parameter and is a measure of a catalyst's ability to make liquids and waxes while making less gas. A catalyst with an α of 0.9 or higher and methane selectivity below five percent is preferred for FTS. Bulk iron catalysts, i.e., iron catalysts having an iron content, calculated as Fe2O3, exceeding about 50 weight percent (wt. %) prepared by precipitation are preferred catalysts, as compared to bulk iron catalysts prepared by fusion, or to supported iron catalysts prepared by impregnation of iron onto a support because of the high activity and selectivity of the precipitated bulk iron catalysts. Preparation of precipitated bulk iron catalysts for FTS has been extensively reviewed [Dry, (1980); Anderson (1984); Lang, X., et al., Industrial and Engineering Chemistry Research, Vol. 34, p. 73, (1995)]. They are typically prepared using iron nitrate as an iron oxide precursor. Copper, (Cu), potassium, (K), and/or SiO2 are added as reduction, chemical, and textural promoters, respectively. The addition of potassium results in a higher α catalyst.
Catalyst attrition is currently a major obstacle to industrial application of precipitated bulk iron catalysts in a SBCR [Bhatt, et al. Proceedings of the 1997 Coal Liquefaction and Solid Fuels Contractor Review Conference, U.S. Department of Energy (DOE), Pittsburgh, Pa., p. 41, Sep. 3-4, 1997; Srinivasan, R et al., Fuel Science Technology International, Vol. 14, p.1337, (1996)]. The non-uniform particles and, especially, the irregular shapes of the catalyst particles produced by precipitation lead to production of catalyst fines by abrasion during use. In turn, attrition of iron catalysts causes (i) plugging of filters, (ii) difficulty in separation of liquid/wax product from the catalyst, and (iii) steady loss of catalyst fines from the reactors.
A number of recent patents [Chaudhary, V. R. et al., U.S. Pat. No. 5,744,419 (1998); Gangwal, S. K. and Jothimurugesan, K., U.S. Pat. No. 5,928,980 (1999); Espinoza, R. L. et al., U.S. Pat. No. 5,733,839 (1998); Rivas, L. A. et al., U.S. Pat. No. 5,710,093 (1998); Moy D., U.S. Pat. No. 5,569,635 (1996)] are directed to the preparation and use of attrition-resistant, supported iron and other metal catalysts for FTS and other processes. Although the use of catalyst supports such as alumina (prepared as spheroids by spray drying) can improve catalyst attrition resistance, supported iron catalysts are generally limited to an iron oxide content of less than 30 wt. %, and have been found to have much lower activity compared to bulk iron catalysts for FTS [Dry, (1980); Anderson, (1984); Bukur, D. B., et al., J. Catalysts, Vol. 29, p. 1588, (1990)]. This is because much less iron is available per unit weight of catalyst. The supports also inhibit the activity of promoters and iron reduction and, thus, reduce catalyst effectiveness.
Improving the attrition properties of bulk iron catalysts is particularly difficult because bulk iron catalysts in a FTS SBCR are subject to both physical attrition and chemical attrition. Physical attrition can be caused by particle collision with other particles or reactor walls and by rapid sparging of gas around the particles. Chemical attrition can be caused during catalyst pretreatment and/or during FTS by iron catalyst phase changes (Fe2O3→Fe3O4→Fe metal and/or Fe carbide), resulting in a decrease or complete loss of physical integrity of the catalyst particles.
Although chemical attrition during the pretreatment of precipitated bulk iron catalysts and during FTS, is not clearly understood, it is well known that the active iron phase for FTS is an iron carbide [Srivastava, et al., Hydrocarbon Processing, (1990); Rao, V. et al., Fuel Processing Technology, Vol. 30, p. 83, (1992)]. The common pretreatment conditions employed are H2 reduction, CO reduction, or syngas reduction with the later two resulting in a more active and higher α catalyst. At least five forms of iron carbides are known to exist; three octahedral-carbides with carbon in the octahedral interstices, and two trigonal prismatic-carbides with carbon in trigonal prismatic interstices. Although the role of these carbides in FTS is not resolved, the multiplicity of carbide phases and iron oxidation states can cause grain boundaries to grow during FTS which can place significant stresses on the iron particle that can lead to chemical attrition.
Spray drying using an appropriate binder is the industrial method of choice for producing microspheroid (40-120 μm) attrition resistant fluidized catalytic cracking (FCC) catalysts and fluidizable alumina in large quantities. It consists of first producing a slurry of catalyst precursor dispersed in a solution of the binder oxide precursor that will form the hard phase of the catalyst [Stiles, A. B., Catalyst Manufacture, Marcel Dekker, Inc., NY, (1983)]. The oxide material must be in the form of discrete colloidal particles. The slurry is then spray dried to form “green” microspheres, mostly larger than 40 μm and mostly smaller than 120 μm that are calcined (heat treated in air) at an appropriate temperature (typically 300-500° C.) to produce attrition-resistant micro-spheroid particles.
Typically, attrition-resistant particles produced by spray drying require 25 to 50 wt. % binder constituting a continuous framework in which are embedded small particles of the active catalyst. Some binders typically used in industry include colloidal silica, colloidal alumina, kaolin clay, and phosphate-modified clay. Bergna, U.S. Pat. No. 4,849,539, (1989); Bergna, U.S. Pat. No. 4,677,084, (1987); and Bergna, H. E. et al., Catalysis Today, 1, p. 49, (1987); disclose a process for producing spray dried, attrition-resistant vanadium oxide/phosphorous oxide catalysts having a lower binder content, preferably about 10 wt % silica-based binder, wherein the binder is added in the form of subcolloidal size particles. During the spray drying process, the subcolloidal size particles of the binder migrate between the spaces of the much larger particles of catalyst or catalyst precursor, to the surface of the spray dried particles and form a hard peripheral composite exterior shell after sintering.
Past attempts to produce attrition-resistant, precipitated bulk iron FTS catalyst microspheres by spray drying have met with failure [Srinivasan et. al. (1996); Bhatt et al., (1997); O'Brien et al., Coal Liquefaction and Gas Conversion Contractor's Review Conference, DOE, (1995)]. In fact, attrition was so severe for a spray dried, high α iron FTS catalyst prepared by United Catalysts, Inc. that a FTS pilot plant at Laporte, Tex. operated by Air Products for DOE had to shut down after only a few hours of testing due to production of catalyst fines and filter plugging [Private Communication with DOE, (1999)].
Espinoza et al, PCT Application WO99/49965, (1999) claim that attrition resistance of precipitated iron FTS catalysts can be increased simply by heat treatment at temperatures above 300° C. without the use of spray drying or binders. However, it is well known that nearly all heterogeneous catalysts, including precipitated iron FTS catalysts, are calcined at 300° C. or higher [Jothimurugesan et al., (1998); Gormley, R. J., et al., Applied Catalysis A: General, Vol. 161, p. 263, (1997)]. Espinoza et al. do not present any attrition results of carbided or used catalysts. Benham et al, U.S. Pat. No. 5,504,118, (1996) teach the preparation of a 5 to 50 μm size iron FTS catalyst for slurry-phase FTS by spray drying without the use of binders. However, these catalysts are not said to be attrition resistant. Thus, such catalyst would not suitable for a slurry-phase reactor from an attrition standpoint and, in addition, the catalysts particles in the lower portion of the 5 to 50 μm particle size range would be likely to plug filters through which wax is removed from the reactor.
Thus, despite substantial effort and research, there are no commercially available precipitated bulk iron FTS catalysts, which are attrition resistant and have substantial catalytic activity. Accordingly, in practice, commercially available precipitated bulk iron catalysts, such as the standard Ruhrchemie pelletized catalyst, are supplied in pelletized form and are limited to use in fixed bed reactors. Nevertheless, precipitated bulk iron catalysts remain the preferred FTS catalysts for low H2/CO ratio syngas processes due to their high activity and selectivity.